March 20 (Reuters) - Louisiana has banned the sale and use of eight compounds marketed as synthetic marijuana to stem an increase in the use of the drugs that have sent scores of people to hospitals with dangerous reactions to toxins, officials said on Thursday.

In recent weeks, more than 100 people have been treated in emergency rooms in the state after using synthetic marijuana, including many suffering life-threatening maladies, they said.

"This emergency rule immediately empowers law enforcement officials to remove these dangerous substances from commerce," said Kathy Kliebert, the chief of the state's Department of Health and Hospitals.

The drugs, also known as synthetic cannabinoids, first began appearing in the United States in 2009, the American Association of Poison Control Centers said.

The National Institutes of Health said that since many chemicals used in synthetic marijuana were then banned by the U.S. Drug Enforcement Administration, manufacturers have substituted other chemicals in their mixtures, causing some users to experience toxic reactions.

"These are untested chemical concoctions that attempt to mimic marijuana without any consideration for the effects these chemicals may have on the human body," law enforcement officials in Louisiana and the coroner for East Baton Rouge Parish said in a statement.

Sold under names street names such as "Spice," "MOJO" and "White Widow", synthetic marijuana has gained a foothold among users who believe it is harder to detect in drug tests.

Synthetic marijuana products are marketed and sold as "safe" alternatives to marijuana, but they can cause extreme anxiety as well as reduce blood supply to the heart, according to the National Institute on Drug Abuse (NIDA).

Synthetic marijuana has been easy to buy in head shops, gas stations and over the Internet, and it is the second most abused illegal substance by high school seniors behind marijuana, according to NIDA.

Synthetic marijuana was thought to be the culprit last year in Colorado that caused the deaths of three people and made another 75 ill, officials in that state said. (Reporting by Jon Herskovitz; Editing by Cynthia Johnston and Cynthia Osterman)